1. Field of the Invention
The present invention relates to a tunable magnetic filed amplifying device; and, more particularly, to a magnetic field amplifying device capable of easily adjusting resonance frequencies and varying usable bands by using discrete elements to vary electric properties of elements used for amplifying a magnetic field of a specific microwave band.
2. Description of Related Art
Magnetic Resonance Imaging (MRI) widely used in the diagnosis of disease is a high-tech non-destructive, non-radioactivity inspection method that is excellent in efficacy but causes no load to the human body. The MRI inspection method applies a static magnetic field of about 0.5 Tesla or greater to a target region of a body. Then, protons in the body have a resonance frequency proportional to the strength of a static magnetic field at about several tens of MHz band or higher. When an electromagnetic wave of such resonance frequency is irradiated in the shape of a pulse to the body, protons absorb the energy of the wave and become excited. After a certain period of time, the protons emit electromagnetic waves of the same frequency and lose energy, thereby going down to a low energy state. The wave being generated at this time is called a Magnetic Resonance (MR) signal. The MRI inspection method then receives this MR signal through a receiving antenna and creates 2-D images representing the density distribution of protons.
In general, higher MR signal intensity results in better resolution of MRI images effective for the diagnosis of diseases. The most direct way to increase the MR signal intensity is to increase the strength of a static magnetic field to be applied. According to the law of thermodynamics, protons in human body that actually absorb energy from the external exciting waves and generate MR signal while losing energy are approximately a part per million (10−6) of the total number of protons. Since this ratio increases proportionally to the intensity of the static magnetic field, the early MRI equipment mainly used permanent magnets that generate the static magnetic field of 0.5 Tesla or below, but the recent MRI equipment employs superconducting magnets that generate the static magnetic field of 1.5 Tesla or greater. However, the MRI equipment using superconducting magnets, compared to the MRI equipment using permanent magnets, is expensive in individual unit price and maintenance cost, which adds the economic burden to many patients.
Another way to improve the resolution is using contrast media or contrast agents for MRI. In general, contrast media are widely used to improve the resolution for X-ray inspection. MRI contrast media are also actively researched now because they can increase the resolution without placing so much cost burden on the patients. However, the MRI contrast media are used in a drug delivery way, so they may cause inconvenience to patients and it may take some time until they are actually used.
Still another way to improve the resolution is to minimize the electromagnetic loss until a receiving antenna detects MR signals generated from the body. The MR signal is essentially an electromagnetic wave of a specific frequency, and composed of two vector components, an electric field vector and a magnetic field vector. But, unlike typical waves traveling in air or vacuum space environment, the MR signal is characterized that the magnetic field strength has a predominant role over the electric field strength. Therefore, a kind of electrical resonant circuit called Swiss roll, which is very sensitive to a change in magnetic field at a specific frequency, is arranged between the body and a receiving antenna in order that a loss in the MR signal can be reduced. The Swiss roll, which is a magnetic field amplifying device, consists of a central mandrel upon which a spiral metal sheet coated with dielectric is wound in the shape of a cylinder N times, so that when an external magnetic field corresponding to its resonance frequency is inputted, it causes an electromagnetic resonance to amplify the magnetic field strength.
Therefore, reducing a loss in the MR signal by using the resonance phenomenon of the Swiss roll, which is the magnetic field amplifying device, has a merit that patients do not have economic burden or feel inconvenient. However, since self-inductance and capacitance to determine the resonance frequency of the Swiss roll is determined by the geometrical shape, it is very difficult to accurately match the resonance frequency to the MR signal from protons. In addition, although Swiss rolls are manufactured under the identical conditions, mass produced Swiss rolls do not always have a uniform resonance frequency due to the tolerance for error. Moreover, physical property changes of individual Swiss rolls caused by temperature, humidity, etc., give rise to the deviation of the resonance frequency, which cannot be corrected easily.